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Solar Cell Industry

In the context of discussions on the expanded use of renewable energy, solar technology has experienced enormous growth around the world in recent years. The overall output of all connected photovoltaic systems already reached 1 gigawatt in Germany alone in 2005, and the tendency was increasing.

As with semiconductor production, silicon is also the dominant material in this field. Alternatives include other anorganic semiconductors such as GaAs or CuInSe or organic semiconductors. However, contrary to semiconductor technology, polycrystalline silicon is especially utilised in addition to single crystalline silicon. This achieves a lower degree of efficiency, but production is considerably less expensive. Amorphous silicon is also used in thinly-layered solar cells. Monocrystalline Si is, as in semiconductor technology, obtained by drawing crystals out of a melt (Czochralski method) and then sawing the resulting product. Polycrystalline silicon is cast in blocks and then sawn. Wafer thickness is always approx. 018 – 0.28 mm. Moreover, other processes exist in addition to the EFG method (edge-defined film-fed growth) or string ribbon process for polycrystalline silicon which involve direct manufacture of the wafers, dispensing with sawing. In addition to the technology involved, only the purest silicon is utilised, as material purity is of decisive importance for the performance of solar cells.

The materials utilised in the production of Si wafers must for this reason meet maximum requirements with regard to their purity. This naturally applies to all parts and components used during the following process steps for further processing of wafers. These components should also exhibit a high degree of temperature and chemical resistance and good thermal conductivity. Due to these reasons, the solar industry utilises practically only quartz glass, graphite, CFC composite materials and coated ceramics. Crucibles, for example, which are used to hold the melted ultra-pure silicon during crystal drawing or casting are made of quartz glass and supported by crucibles manufactured from carbon fibre reinforced carbon. Graphite or CFC heating elements fixed with CFC screws are used for heating, due to their extremely high purity. Evaporators utilised for applying coatings after wafer production are also made of these materials.

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